There have been devised balance-unbalance conversion elements that have one ½-wavelength resonator and two ¼-wavelength resonators formed on a dielectric substrate and performs balance-unbalance conversion (for example, see Patent Document 1).
FIG. 1 shows a related-art example where a balun is formed as a balance-unbalance conversion element. A balun 101 is formed by laminating multiple dielectric substrates. The balun 101 includes ground electrodes (not shown) provided on a top surface A and a bottom surface B, an unbalance terminal (not shown) provided on a left side surface C, and two balance terminals (not shown) provided on a right side surface D. An unbalance pattern 102 is provided on an illustrated top surface of a substrate 105. The unbalance pattern 102 is an electrode forming a ½-wavelength resonator. Also, balance patterns 103A and 103B are provided on a dielectric substrate laminated on the back surface of the dielectric substrate 105. The balance patterns 103A and 103B are electrodes forming different ¼-wavelength resonators.
The unbalance pattern 102 is an approximately U-shaped electrode including line portions 102A and 102B disposed in parallel, a line portion 102C coupling the line portions 102A and 102B, an extended electrode 102D to be coupled to a ground electrode, and an extracting electrode 102E to be coupled to an unbalance terminal. The balance patterns 103A and 103B are approximately I-shaped electrode patterns. The line portions 102A and 102B of the unbalance pattern 102 are each opposed to the balance pattern 103B or 103B with a first dielectric substrate therebetween.
When an unbalance signal is inputted into the unbalance terminal of the balun 101, the balun 101 converts the unbalance signal into a balance signal, outputs a first balance signal from one balance terminal thereof, and outputs a second balance signal having a phase almost opposite to the phase of the first balance signal from the other balance terminal thereof.
Conversely, when balance signals are inputted into the two balance terminals of the balun 101, the balun 101 converts the balance signals into unbalance signals, and outputs the unbalance signals from the unbalance terminal.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 10-290107.
In general, balance characteristics of a balance-unbalance conversion element is evaluated using the amplitude of the frequency band where the phase difference and amplitude difference between two balance signals fall within respective desired ranges.
However, in the balun 101, which is a related-art example, the shape of the unbalance pattern 102 and the disposition of the balance patterns 103A and 103B are asymmetrical. For this reason, the balun 101 has a problem that the frequency band where proper balance characteristics can be obtained is narrow.